The need for denser, larger and more durable chip assemblies has broadened the use of Direct Chip Attach (DCA) technology to include flip chip integrated circuits. A typical flip chip integrated circuit utilizes a solder ball grid array to provide electrical connections between a die of the flip chip and a substrate. During manufacturing of a typical flip chip, after the flip chip is assembled on a substrate, a liquid dispensing system is used to apply an underfill encapsulant material between the die and the substrate. The flip chip underfill material is used to reduce mechanical and thermal stress on the electrical connections and to protect the electrical connections against atmospheric conditions. The underfill material provides stability and rigidity to the assembled flip chip and may also be used as a heat conductor to improve thermal performance of the flip chip.
In typical prior art flip chip underfilling processes, a dispenser system is used to dispense underfill material around the sides of the flip chip and the underfill material spreads under the flip chip and around the solder balls of the grid array via capillary action or "wicking". During the assembly process, the substrate is typically heated prior to, during, and after dispensing of the underfill material to a temperature ranging from ambient conditions to approximately 120.degree. C. The heating of the substrate increases the capillary action causing the underfill material to flow further under the die of the flip chip. A final fillet of underfill material is applied around the sides of the flip chip after the wicking action has occurred.
A drawback associated with underfilling processes is that the underfill material may not completely fill all voids between a die and a substrate in a flip chip. The result of this problem can be seen in FIG. 1, which shows a cross-sectional view of a flip chip die 10 connected to a substrate 12 through a number of solder ball connections 14. In FIG. 1 underfill material 16 has been applied around the edges of the substrate and die and the underfill material has partially filled the area between the die 10 and the substrate 12 leaving a void 18 containing no underfill material. The void 18 may affect the performance and stability of the flip chip, and thus, flip chips like that shown in FIG. 1 having voids are typically rejected by manufacturers resulting in many wasted parts. Defective chips having voids which escape detection are more likely to exhibit operational failures during use.
To overcome the problem of voids or air gaps, one prior art dispensing system developed by Tessera of San Jose, Calif. utilizes a vacuum approach to completely underfill flip chips. In this prior art system, the dispensing system, including one or more flip chips that are to receive underfill material, is enclosed within an air tight chamber, and prior to the dispensing of underfill material, a vacuum pump is used to purge all air from the chamber to create a vacuum. The underfill material is then dispensed around all sides of the flip chips, and the chamber is returned to ambient pressure. When the chamber is returned to ambient air pressure, the underfill material is forced under the flip chips by the difference in air pressure outside the flip chips and under the flip chips.
While the above described prior art system is effective in preventing voids in underfill material in flip chips, the system is relatively large and the time required to purge air from the air tight chamber is rather long. Further, because the air tight chamber is so large, it is difficult to effectively purge air from the chamber. In addition, the air tight chamber of the prior art accommodates only manual loading of the flip chips into the chamber, preventing the dispensing system contained within the chamber from being effectively used in an automated assembly line.